1. Field of Invention
This invention relates to a device and process for thermal treatment of waste gases and reactive gases. The invention is used for the destruction and reduction of pollutants from effluent waste streams, and to produce gaseous products from reactant gases. This invention more specifically relates to a coated susceptor of electromagnetic energy for chemical processing in which a matrix material surrounds a non-matrix material that is made from a material that is different from the matrix material, and the matrix material is constructed of material having lower dielectric losses compared to the non-matrix material.
2. Prior Art
Devices, which operate on electricity to thermally treat gases from waste streams to reduce pollution and thermally react gases for synthesis of products, do not rely on natural gas for supplying energy. Devices that use natural gas to produce energy for such applications create carbon dioxide, carbon monoxide and nitrogen oxides from the energy source. Electricity is considered to have cleaner operation when used in such devices since the above chemical species are not produced during operation from the heat source. Electric devices for pollution control applications produce less pollution at the point source when compared to the counter technologies operating on natural gas. The reduced pollution is favorable to reduce greenhouse gases and to the meet the requirements of the Clean Air Act of 1990. There are many types of electric heating methods; this discussion will focus on designs used to produce heat and reaction with applied electromagnetic energy.
The scope of this current invention is a device for thermal treatment of gases and pollutants that employs 1) alternate cavity and susceptor geometries for providing more homogeneous interactions of applied electromagnetic energy in the volume of the susceptor regardless of the flow rate and diameter of the exhaust duct width, 2) heat transfer methods to improve the overall heat efficiency of the device, 3) a susceptor structure that has reflectivity as principle mode of interaction with applied electromagnetic energy, which allows for energy to penetrate a susceptor, 4) composite susceptor materials, 5) a simple method of controlling the temperature versus energy concentration in the susceptor, and 6) field concentrators to concentrate the energy density of the applied electromagnetic energy.
Cavity geometries in these devices affect the optical properties of the electromagnetic energy within the susceptor. Electromagnetic energy, whether ultraviolet, infrared, microwave or radio frequencies, exhibits the same optical properties as the visible spectrum when interacting with geometric shapes and surfaces that are similar to a lens. The electromagnetic energy in a susceptor can either converge or diverge due to the geometric shape of the susceptor following the same principles as optical lenses. Additionally, the modes of propagation of the electromagnetic energy are dependent upon the cavities geometry. These modes effect the distribution of electromagnetic energy in the cavity. These modes are different for cylindrical and rectangular cavities (see, e.g., Handbook of Microwave Engineering).
Electromagnetic energy that is incident perpendicular to the perimeter of the circular cross-section of a cylindrical susceptor will converge initially, concentrating the energy within the cross-section. This concentration will cause the material inside the susceptor to absorb more energy than the material near the surface, changing the dielectric properties of the material inside the cross-section. This concentration of energy can make the material, which is located in the susceptor's interior, between the center and the perimeter, to absorb more energy, thereby reducing the depth of penetration of the material due to the susceptor's geometry.
The optical properties of rectangular cavities and planar surfaces are different. Rectangular cavities with a susceptor having a rectangular geometry and planar surfaces will follow the optical properties of a flat surface. A flat surface does not concentrate or disperse energy as do curved surfaces, such as convex and concave surfaces. With a flat surface of incidence for applied electromagnetic energy, the absorption of electromagnetic energy in a susceptor is due only to the properties of the materials and is not influenced by energy, which is concentrated by curved geometries. Incident energy on susceptors with flat surfaces will not be concentrated within a structure with homogeneous material, and the depth of penetration will be influenced by the incident energy's power, the electric fields and magnetic fields inside the susceptor. Conversely, incident energy on susceptors with curved geometry can be concentrated within a susceptor with homogeneous materials, and the depth of penetration of the energy will be influenced by the ability of the curved surface to concentrate energy inside the susceptor.
The overall energy efficiency of such devices for thermal treatment of gases can be improved with a better heat transfer process to capture the energy that is lost from cooling the tube that is the source for the applied electromagnetic energy. In industrial microwave drying operations, the heat produced from cooling the magnetrons with air is applied to the articles that are being dried with the microwaves. This synergistic drying, which uses hot air and microwaves, increases the energy efficiency of the drying process.
Alternative composite materials and susceptor structures can be used to facilitate the thermal treatment of gases. These composite materials and susceptor structures are known as artificial dielectrics.
Artificial dielectric structures date back to the 1940s. Artificial dielectrics were used as lenses to focus radio waves for communication (Koch). Artificial dielectrics use conductive metal plates, rods, spheres and discs (second phase material) which are embedded in matrices of low dielectric constants and low dielectric losses to increase the index of refraction, thus reducing size of a lens to achieve the desired optical properties. The second phase material reflects the energy and uses diffuse reflection to transmit electromagnetic energy. These plates, rods, spheres, and discs can be arranged in a lattice structure to produce an isotropic or an anisotropic structure.
When conductive elements are embedded in a low dielectric constant and low dielectric loss matrix, the effect of these on the matrix material's dielectric loss factor is negligible and the dielectric constant of the composite lens is increased. However, these above effects are limited and influenced by the size, shape, conductivity and volume fraction of the material embedded in a matrix of low dielectric loss, low dielectric constant of the material as well as the wavelength of the incident radiation. The dielectric strength and complex dielectric constant of the matrix material plays important additional roles in the design of artificial dielectric lenses. On the other hand, selection of matrix materials with different dielectric properties and incorporation of second phase materials such as semiconductors, ferroelectrics, ferromagnetics, antiferroelectrics, antiferromagnetics, dielectrics with higher dielectric losses, and dielectrics with conductive losses that produce absorption of microwave energy, produce heat in an artificial dielectric.
Lossy artificial dielectrics were demonstrated by the 1950s and subsequently used at the microwave frequencies to sinter ceramic articles, in food packaging for heating foodstuffs, in browning apparatuses for foodstuffs, in consumer products, and to render adhesives flowable for bonding applications.
The structure of the artificial dielectrics determines the electromagnetic properties. When the volume fraction of the second phase materials inside the artificial dielectric reaches a certain level, the artificial dielectric will reflect incident electromagnetic energy, shielding the artificial dielectric from absorbing electromagnetic energy. The volume fraction of the second phase material at which the artificial dielectric shields electromagnetic energy is dependent on the second phase material's reflectivity, the shape of the second phase material, and the temperature. By controlling the amount of reflection, the susceptor's reflectivity can be used to control the susceptor's temperature.
Reflectivity has been used to produce structures that have a self-limiting temperature. Producing reflectivity in dielectrics is explained in Von Hipple's Dielectrics and Waves. Using such principles, devices have been designed to have self-limiting temperatures. Self-limiting temperatures have also been theorized for materials with Curie temperatures. The reflectivity of electromagnetic energy is related to a material's conductivity. Metals are electrically conductive at room temperatures and reflective of electromagnetic energy. Semiconductors and ionic conductors have low moderate conductivity at room temperature. At elevated temperatures semiconductors and ionic conductors have increased conductivity, and these materials will become reflective to electromagnetic energy at elevated temperatures. The amount of reflectivity of a material at elevated temperature will also be dependent upon the wavelength of incident electromagnetic energy.
The artificial dielectrics structure can be used to produce diffuse reflection, or scattering, inside a susceptor. The second phase materials either can be reflective materials at room temperature, such as a metal, or can become reflective at elevated temperatures due to 1) increasing conductivity, such as semiconductors and ionic conductors and/or 2) exceeding the Curie temperature, such as ferroelectrics and ferromagnetics. This diffuse reflection may also be used to control the temperature of a given susceptor that uses the artificial dielectric structure.
Regardless of the structure of a susceptor and its materials of construction, applied energy must be applied to penetrate the structure and material or materials of construction for volumetric interaction between the susceptor and the applied energy.
Other considerations must be given to the structure of a susceptor in a device for thermal treatment of gases. Honeycombs, foams, packed material and woven structures, which are constructed of a material that either has an increased dielectric conductivity at elevated temperatures or has a Curie temperature below the operating temperature could become reflective. If the material becomes reflective, then the susceptor's structure either could a) act as waveguides with dimensions that would not allow the applied energy to penetrate because the applied energy would be below the cut-off frequency for the susceptor's structure or b) shield the electromagnetic energy from penetrating into the susceptor. The Handbook of Microwave Engineering Handbook explains waveguide theory in more detail. For example, granular suscepting structures employed in U.S. Pat. No. 4,718,358 for treatment of gases exemplify conditions where the susceptor's structure may not allow for incident electromagnetic energy penetrate the volume of the susceptor.
It seems to appear that the authors of U.S. Pat. No. 4,718,358 preferably embody granular absorbing material in the range of about 5 mm to 10 mm with a layer thickness, which is preferably 100 mm to 300 mm. One of the preferred absorbing materials is SiC in granular form. Silicon carbide, a semi-conducting ceramic, has a moderate penetration depth of approximately 10 cm at room temperature. And, depending upon the purity of the SiC, the depth of penetration can be less then 2 cm at room temperature. At elevated temperatures, silicon carbide becomes more conductive, thus having an even lower penetration depth. If one assumes that the granules in U.S. Pat. No. 4,718,358 are spherical, then the 10 mm spheres of the SiC would most likely pack inside the cylindrical cavity in what is known as the close-packed cubic structure. The close-packed cubic SiC structure would have a void volume of only 26%. The largest void space in this granular pack of 10 mm SiC spheres in the close-packed cubic structure would be occupied by what is known as an octahedral site. The octahedral site is the void space between six spheres—four spheres touching in one plane, one on the top of and one on the bottom of the void space formed between the four-spheres touching in one plane. The void diameter of the octahedral site at the largest diameter would be about 6 mm. With an open space of the 6 mm in width and the device in U.S. Pat. No. 4,718,358 operating at approximately 900° C., where the dielectric conductivity of SiC is greatly increased in comparison to the dielectric conductivity at room temperature, one can question the ability of the microwave energy at 2.45 GHz and wavelength of approximately 13 cm to propagate through the close-pack cubic structure of the SiC granules and heat a volume of SiC with a depth of the particles being between 100 mm to 300 mm. Does the packed SiC spheres at the operating temperature of 900° C. act as a collection of small waveguides that have dimensions below the cut off frequency for the applied electromagnetic radiation? If so, the susceptor's structure will not allow for the applied energy to penetrate into the entire volume of SiC granules. This type of structure would shield electromagnetic energy as exemplified in common practice by windows of household microwave cooking ovens. Or, does the packing of SiC spheres at an operating temperature of 900° C. have a finite depth of penetration that neither allows for the volumetric heating of the entire mass of SiC granules nor has electromagnetic energy throughout the volume of the SiC mass to interact with gaseous species for possible enhanced reactions? This latter argument for a finite depth of penetration in this susceptor arrangement would most likely heat a finite volume of SiC granules near the surface of the incident applied radiation, then heat would be thermally conducted through the SiC to the remaining volume of SiC granules since SiC is a very thermally conductive material. One could argue that a greater power level of applied electromagnetic energy could be incident on the SiC granules in an attempt to heat the entire volume, however depth of penetration can become less at increased levels of applied power. The greater power level will cause the depth of penetration to migrate to the surface where the applied electromagnetic energy is initially incident upon, when the SiC material becomes more conductive at elevated temperature. The increased conductivity can cause the material to become reflective to the applied energy.
Other suscepting structures such as honeycombs, foams and woven structures can have similar concerns about the depth of penetrations as mentioned above. These structures, when made of semiconducting, conducting, ferromagnetic, ferrimagnetic, ferroelectric and anitferroelectric materials, can have shallow depths of penetration. Graphite, carbon black, magnetite (Fe3O4), MnO2 are materials that have depths of penetration less than 1 mm at room temperature. When suscepting structures, such as honeycombs, foams and weaves are coated with these material, the structures either will have shallow penetration depths or will act as waveguides that have dimensions that are below the cutoff frequency regardless of a) the bulk material or materials that makes up the substrate for the coating and b) the design of the susceptor's structure. Consequently, a susceptor must be properly designs for volumetric interaction with the electromagnetic energy, taking into consideration the materials of construction, the structure and the effects of coatings.